1. Field of the Invention
The present invention relates to a gas turbine power plant control method and equipment, and more specifically to a method and equipment for controlling start and stop of an electric power plant provided with a gas turbine.
2. Description of the Prior Art
FIG. 12 is a schematic system diagram showing an electric power plant provided with a gas turbine, in which rotary bodies of a gas turbine 1, a compressor 2 and an electric power generator 3 are all fixed to a signal turbine shaft.
When the compressor 2 is rotated, air is introduced into a combustor 4, and then burnt together with fuel within the combustor 4, to generate a combustion gas. The generated combustion gas is supplied to the gas turbine 1, so that a power can be outputted from the turbine shaft. In this case, the flow rate of air supplied to the compressor 2 can be controlled by changing the opening rate of an inlet guide vane 5 disposed on the inlet side of the compressor 2.
In general, the construction such that the gas turbine 1, the compressor 2 and the electric power generator 3 are all coupled to a single gas turbine shaft is called a shaft structure. In the above-mentioned shaft structure, an auxiliary steam system and a starting equipment are provided in addition to the gas turbine 1, the compressor 2 and the electric power generator 3. Further, in general, a single power system is constructed by a plurality of shaft structures, and one electric power plant is constructed by arranging a plurality of the electric power systems. However, there exists such an example that an electric power plant is constructed by a single power system.
In the gas turbine electric power plant, in general the turbine shaft is rotated at a low speed as a preparatory operation, before the turbine shaft is started, which is referred to as turning operation. Here, the procedure from the low speed revolution to the rated speed revolution of the turbine shaft is as follows: in the case where a gas fuel is used as the fuel, in order to protect the power system from the unburnt fuel remaining on the downstream side of the gas turbine, purge operation for introducing air from the air compressor 2 to the gas turbine 1 is performed for 5 to 15 min by rotating the turbine shaft at a revolution speed of about 20 to 40% of the rated revolution speed. In the case where a liquid fuel is used, however, this purge operation can be omitted.
Further, until this purge operation ends, a necessary torque is kept generated to rotate the turbine shaft by use of a starting equipment.
In the state where this purge operation ends, since the revolution speed of the turbine shaft is higher than a predetermined revolution speed thereof required to ignite the gas turbine, the revolution speed of the turbine shaft is reduced from that for the purge operation to that for the gas turbine ignition, by controlling the revolution speed of the turbine shaft by use of the starting equipment. Further, after having been ignited, the gas turbine is warmed and then shifted to a speed-up control.
In this speed-up control, the revolution speed of the turbine shaft is increased in such a way that predetermined acceleration can be generated according to the revolution speed of the turbine shaft in accordance with a function. Further, a torque to be outputted according to the shaft revolution speed is set to the starting equipment. Therefore, an output torque of the starting equipment is subtracted from the acceleration torque required to generate the determined acceleration, and the amount of fuel to be supplied to the combustor 4 is controlled in such a way that this differential torque can be obtained by the gas turbine. In other words, the revolution speed of the turbine shaft is controlled indirectly.
As described above, in the prior art method, since the power required to drive the compressor exceeds the power generated by the gas turbine at the start of the turbine shaft, the starting equipment has been used as another driving source for starting the gas turbine.
Here, the acceleration of the gas turbine is so determined as to avoid an unstable operation of the compressor and further to prevent an excessive temperature rise of the combustion gas on the outlet side of the combustor. In particular, since the temperature of the combustion gas is determined below a level, the maximum capacity of the starting equipment is determined on the basis of the level of the combustion gas temperature.
As described above, after the gas turbine has been ignited, the revolution speed of the turbine shaft is increased up to near the rated revolution speed by the torque generated by the gas turbine and the torque supplied by the staring equipment. Here, when the revolution speed of the turbine shaft reaches roughly the rated revolution speed, the starting equipment is separated from the turbine shaft. Therefore, after that, the operation of the turbine shaft is controlled by the control equipment of the gas turbine. Further, after the revolution speed of the shaft has reached the rated revolution speed and thereby the electric power generator 3 has been connected to an external power system, the gas turbine electric power plant is shifted to the ordinary operation.
On the other hand, in stop process, the operation of the gas turbine electric power plant is controlled as follows: After the fuel has been reduced rapidly from the base load operation (the maximum load operation of the gas turbine) to the no-load operation, the electric power generator is disconnected from an external power system. After the electric power generator has been disconnected from the external power system, the revolution speed of the gas turbine is not dependent upon the frequency of the external power system. In contrast with this, when the electric power generator is connected to the external power system in the ordinary operation, the revolution speed of the gas turbine is the same as the frequency of the external power system. For instance, in the region of 50Hz, since the revolution speed of the gas turbine is 3000rpm, that is, 50rps. That is, once the electric power generator has been disconnected from the external power system, since the revolution speed of the gas turbine is not dependent upon the frequency of the external power system, it is necessary to control the gas turbine in such a way that the revolution speed thereof will not exceed the rated revolution speed. That is, it is necessary to reduce the revolution speed of the gas turbine by sufficiently reduce the flow rate of the fuel to be supplied to the gas turbine. Here, in the case of the gas turbine of 150MW to 250 MW power class, a time required from the disconnection to the stop is about 5 to 10 min.
In this stop process, temperature drops rapidly at the high temperature sections of the gas turbine (e.g., a combustion chamber, transition pieces, first-stage static vanes, moving vanes, etc.). For instance, in the case of the gas turbine of 1300.degree. C. class, although the inlet side temperature of the first-stage moving vanes is 1300.degree. C. in the base load operation, the same temperature of the first-stage moving vanes drops down to about 700.degree. C. to 900.degree. C. in the no-load operation. Further, in the stop process, since the inlet side temperature of the first-stage moving vanes drops down to about 20.degree. C. to 50.degree. C. (roughly equal to the room temperature). As described above, the high temperature parts of the gas turbine are subjected to an excessive thermal stress, so that the life time of these high temperature parts of the gas turbine is reduced markedly.
In particular, the high temperature parts of the gas turbine are usually casted precisely by use of an Ni or Co-based supper alloy in such a way as to form a complicated air cooling structure in the casted parts. Therefore, in the high temperature parts of complicated structure, stress is easily concentrated at various positions thereof. In addition, since these parts are of cast products, the material of the high temperature parts is very weak against the strain caused by thermal stress. Accordingly, in the actual gas turbine now being operated, there exist a problem in that a great number of cracks are easily generated in the high temperature parts with increasing operation time and with increasing number of start and stop times, with the result that it is necessary to repair the high temperature parts periodically by welding, for instance.
In particular, during the operation course from the start to the rated load operation, the strains caused by thermal stress increase in the high temperature parts. For instance, in the case of nozzle, since the nozzle temperature rises from the low temperature at the stop to the maximum temperature in the rated load operation, the material for constructing the nozzle expands. As a result, since the nozzle is fixed to the casing, a compressive strain (a strain caused in the compression direction) is generated. On the other hand, during the operation course from the rated operation to the stop operation, since the nozzle fixed to the casing is cooled rapidly, a tensile strain (a strain caused in the tension direction) is generated.
As described above, since a difference between the compressive strain and the tensile strain causes a one-cycle thermal stress change from the start operation to stop operation, so that a fatigue based upon the thermal stress (referred to as low-cycle fatigue) is repeated. In this case, the compressive strain is subjected to the rated operation at the maximum gas temperature and the tensile strain is subjected to the stop operation at the room temperature, both being subjected to the time change of the gas turbine operation.
At present, a greater number of the domestic gas turbine electric power plants are operated as an intermediate load power source of "daily start and stop"(the daily operation and the night operation are repeated every day). Therefore, since the life time of the high temperature parts of the gas turbine is reduced in the operation change from the start to the stop, the maintenance cost is huge. This is because the rare metal material of Ni- or Co-based supper alloy having a temperature resistance as high as 700.degree. C. to 900.degree. C. must be used as the material for the high temperature parts, and in addition the expensive high temperature parts of precise cast products formed with a number of cavities for air cooling must be repaired by welding or exchanged whenever the gas turbine maintenance is made for each year or ever second year.
In summary, in the prior art gas turbine electric power plant, during the operation course from the gas turbine start to the gas turbine stop, since the temperature change range and the temperature change rate are both large in the outlet gas temperature of the combustor, there exists a problem in that the life time of these high temperature parts for constituting the gas turbine is short.
In addition, in the prior art gas turbine electric power plant, an effort to reduce the temperature change rate within the gas turbine in the stop operation has been so far made by delaying the deignition timing as long as possible, after the disconnection of the electric power generator from the external power system. In this case, however, when the revolution speed of the turbine shaft is dropped after the disconnection, since the efficiency of the compressor drops abruptly and thereby the gas temperature drops at the same time, the fuel supplied to the gas turbine is also reduced, so that the turbine output also drops. As a result, the revolution speed of the gas turbine drops abruptly; the air flow rate also decreases; and thereby the fuel to be supplied decreases. Therefore, when the gas temperature is tried to be reduced gradually, there exists inevitably a limit due to the basic and inherent characteristics of the gas turbine.